1. Field of the Invention
The present invention relates to a sealed battery.
2. Description of the Related Art
In recent years, development and spread of various electrical machines and devices have been encouraging wide use of batteries, especially sealed alkaline storage batteries as power sources thereof. As representative examples of sealed alkaline storage batteries, a nickel-cadmium storage battery, a nickel-hydrogen storage battery and the like can be mentioned.
Generally, batteries of this type have a structure shown in FIG. 1, and produced in the following way: A negative electrode 2′ and a positive electrode 3′ with a separator 4′ between are wound spirally into a spiral-wound electrode assembly 5′. After the spiral-wound electrode assembly 5′ is inserted into an outer can 1 through its opening, an alkaline electrolyte is injected into the outer can 1. Last, the opening of the outer can 1 is sealed with a sealing member 7′.
The positive electrode 3′ comprises a positive electrode substrate and a positive active material layer supported by the positive electrode substrate, while the negative electrode 2′ comprises a negative electrode substrate 2A′ and a negative active material layer 2B′ supported by the negative electrode substrate 2B′. The positive electrode substrate and the negative electrode substrate 2A′ are each made of a metal having good electroconductivity. The positive active material layer and the negative active material layer 2B′ each contains active material, for example, in powder form, and if necessary, a binder for increasing binding strength between powders and between powders and the substrate.
As a negative electrode 2′ of this type, for example, a paste-type cadmium negative electrode is produced in the following way: Negative active material which contains cadmium oxide powder as a main component, and a binder including hydroxy propyl cellulose are mixed to prepare negative electrode active material slurry. The negative electrode active material slurry is applied on both sides of a nickel punching sheet which forms a negative electrode substrate 2A′, and then dried.
Regarding the above-described battery structure, there are two main modes of current collection from the negative electrode 2′, or in other words, two main modes of electrically connecting the negative electrode 2′ and the outer can 1. In a first mode, a current collector prepared separately is welded to the lower end of the negative electrode substrate 2A′ of the spiral-wound electrode assembly 5′, and also welded to the bottom of the outer can. In a second mode, as shown in FIG. 1, the negative active material layer 2B′ which forms the outer circumferential face of the spiral-wound electrode assembly 5′ is brought in direct contact with the inner side face 1a of the outer can 1.
In the second mode, it is not necessary to prepare a current collector and weld it to the spiral-wound electrode assembly and to the bottom of the outer can. Thus, when a battery is produced in the second mode, the production cost can be lower than when it is produced in the first mode. Thus, the second mode is employed in many batteries.
However, if the second mode is employed in the case where the binding strength of the negative active material layer 2B′ is weak, trouble may happen. That is, in the step of inserting the spiral-wound electrode assembly 5′ into the outer can 1, when the negative active material layer 2B′ touches the edge of the opening or the inner side face 1a of the outer can 1, parts of the negative active material layer 2B′ may fall off the negative electrode 2′. This causes problems such as lowering of the battery capacity, and in the worst case, the parts of the negative active material layer which have fallen off short-circuit the negative and positive electrodes.
In order to avoid these problems, it is conceivable to increase the amount of the binder contained in the negative active material layer to thereby increase the binding strength. However, this solution is not desirable, because, in this case, the proportion of the active material contained in the negative active material layer decreases, and the battery capacity decreases accordingly.
In this connection, for example, in the case where a paste-type cadmium negative electrode is employed, there is known a method in which cadmium oxide, which is active material, is changed into cadmium hydroxide to increase the binding strength of powders themselves to thereby increase the binding strength of the active material layer.
However, this method has a problem: In order to increase the binding strength of the negative active material layer, a new step needs to be added to the process of producing a battery, which increases the production cost accordingly.
The above-mentioned problems come from employing the second mode in order to eliminate the cost of preparing a current collector and welding it to the outer can. Thus, in order to avoid the above-mentioned problems, it is conceivable to employ the first mode in which a current collector is used, in place of the second mode. However, when the first mode is employed, increase in production cost is inevitable.
Further, if the second mode is employed in the case where the electroconductivity of the negative active material layer 2B′ is low, there happens a problem: What is in direct contact with the inner side face 1a of the outer can 1 is the negative active material layer 2B′ supported by the negative electrode substrate 2A′. Thus, when the electroconductivity of the negative active material layer 2B′ is low, the contact resistance between the inner side face 1a and the negative active material layer 2B′ is large and varies to a large degree, and accordingly, the internal resistance of the battery is large and varies to a large degree.
However, it is known that, in the case where the electroconductivity of the negative active material layer is low, if, for example, a sponge nickel substrate or a porous sintered nickel substrate is employed as the negative electrode substrate, it compensates for the low electroconductivity of the negative active material layer 2B′. Specifically, since numberless small holes in the sponge nickel substrate or porous sintered nickel substrate are filled with the active material, the sponge nickel substrate or porous sintered nickel substrate provides current paths, to thereby compensate for the low electroconductivity of the negative active material layer.
However, the negative electrode using the sponge nickel substrate has a problem: Since the sponge nickel substrate is costly, the material cost is high. Also, the negative electrode using the porous sintered nickel substrate has a problem: In order to produce the porous sintered nickel substrate, sintering needs to be performed to turn nickel powder, which is a material for the negative electrode substrate, into a porous material. Thus, when the battery is produced using this negative electrode, the production cost increases because of the material cost of nickel powder and the cost of sintering.
The object of the present invention is to provide, at low cost, a sealed battery wherein current can be collected from the negative electrode effectively without using a current collector, even if the binding strength of the negative active material layer is weak, and wherein the internal resistance of the battery is small and does not vary to a large degree, even if the electroconductivity of the negative active material layer is low.